对于已经有值的field,在mongodb中可以使用$unset操作符来重置该field。
http://docs.mongodb.org/manual/reference/operator/update/unset/
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具体语法如下:
{ $unset: { <field1>: "", ... } } 例如 db.products.update( { sku: "unknown" }, { $unset: { quantity: "", instock: "" } } )
具体使用的例子:
db.dbdao_sample.insert({"_id":3,"a":7,"c":[3,4,7,-1,-1]}); > db.dbdao_sample.update({"_id":3},{"$unset" : {"c":""}}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > db.dbdao_sample.find(); { "_id" : 3, "a" : 7 } db.dbdao_sample.insert({"_id":4,"a":7,"c":[3,4,7,-1,-1]}); db.dbdao_sample.update({"_id":4},{"$remove" : {"c":""}}); 没有$remove这个操作符 > db.dbdao_sample.update({"_id":4},{"c": {"$delete":true }}); 2015-05-05T20:38:33.817+0800 E QUERY Error: field names cannot start with $ [$delete] at Error (<anonymous>) at DBCollection._validateForStorage (src/mongo/shell/collection.js:161:19) at DBCollection._validateForStorage (src/mongo/shell/collection.js:165:18) at DBCollection._validateUpdateDoc (src/mongo/shell/collection.js:388:14) at Object.findOperations.updateOne (src/mongo/shell/bulk_api.js:675:20) at DBCollection.update (src/mongo/shell/collection.js:455:22) at (shell):1:17 at src/mongo/shell/collection.js:161 > db.dbdao_sample.update({"_id":4},{"c":null}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > > db.dbdao_sample.find({"_id":4}); { "_id" : 4, "c" : null } > db.dbdao_sample.update({"_id":4},{"c":1}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > db.dbdao_sample.find({"_id":4}); { "_id" : 4, "c" : 1 }